The present invention relates in general to a process of fixing a titanium-lined double-walled tubing structure to a titanium tube sheet, and more particularly to a process by which a double-walled tubing structure comprising a non-titanium outer tube and a thin-walled titanium inner lining tube in pressed contact with the inner surface of the outer tube, is suitably fixed to a titanium tube sheet of an apparatus such as a condenser.
In the art of condensers and heat exchangers, it has been known and attempted to use titanium-lined double-walled tubing or piping structures each of which comprises a non-titanium outer metal tube and a thin-walled, highly corrosion-resistant titanium inner lining tube held in pressed contact with the inner surface of the outer metal tube. For example, there is known a double-walled tubing which is a combination of a finned aluminum outer tube and a thin-walled titanium inner lining tube. The finned aluminum outer tube exhibits high thermal conductivity upon condensation of a working fluid, while the titanium inner lining tube demonstrates high sea-water corrosion resistance. In a condenser using such double-walled tubing structures with the abovementioned properties, there is provided a heat exchanging arrangement wherein sea water or other similarly corrosive fluid (cooling fluid) flows within the internally titanium-lined double-walled tubing, while the outer surface of the tubing is exposed to a stream of a condensable fluid which is not corrosive to an aluminum material of which the outer tube is formed. More specifically, the condensers using double-walled tubing structures of the type described above include: a condenser for power generator using ocean thermal energy (utilizing temperature differential), which employs ammonia or fluoro-carbon as a condensable working fluid; a fluoro-carbon turbine condenser of vertical type; and a liquefying condenser for LNG (liquefied natural gas).
The double-walled tubings used in a condenser are generally mounted on tube sheets which divide the condenser into sections, so that a corrosive cooling fluid such as sea water introduced into these condenser sections flows through the inside of the double-walled tubings fixed to the tube sheets. Since the tube sheets are also exposed to the corrosive cooling fluid, it is preferred that the tube sheets be made of titanium. When titanium tube sheets are used, however, there are some problems that should be solved, associated with the manner of fixing the double-walled tubings to the titanium tube sheets. Described in greater detail, the double-walled tubings are conventionally fixed to the titanium tube sheets, for example, by mechanical diametrical expansion of the double-walled tubing, or by seal-welding technique. These known methods of fixation suffer various drawbacks or inconvenience which follow.
In the case where the titanium-lined double-walled tubing is fixed to the titanium tube sheet by means of mechanical diametrical expansion of the double-walled tubing, each end portion of the tubing inserted in a mounting hole in the tube sheet is mechanically expanded in diameter by suitable means so that the outer surfaces of the tubing are held in pressure contact with the inner surface defining the mounting hole. The fixation by the mere pressed contact between the two members is not sufficient to assure or maintain satisfactory levels of fluid tightness, thermal conductivity, and a reliably high degree of fixing strength. Further, this manner of fixation has a problem that the ends of the outer tube made of aluminum or other metal materials with relatively low corrosion resistance are exposed to the corrosive fluid flowing into the double-walled tubing, whereby the end portions of the tubing tend to be corroded by the fluid. Accordingly, it is necessary to coat the end faces of the double-walled tubing with a suitable anti-corrosion paint, or cover the ends of the tubing with suitable protective caps.
On the other hand, where the double-walled tubing is welded to the tube sheet, the end portion of the titanium-lined double-walled tubing is inserted in the mounting hole in the tube sheet such that a titanium sleeve is interposed between the outer surface of the tubing and the inner surface of the mounting hole, and the end of the double-walled tubing is fusion-welded along its circumference in a seal-welding process. In this manner of fixation, too, the welded end of the tubing made of aluminum or other non-titanium metal with low corrosion resistance is subject to corrosion by the corrosive working fluid. Accordingly, the welded ends of the tubing must be protected as with suitable covers as previously indicated. This protective covering will lead to increase in number of assembling steps, and in assembling time and cost.